According to a known electric power steering device for motor vehicles, the motor shaft consists of a hollow shaft, and a drive shaft connected to the right and left steerable wheels via tie rods is coaxially received in the motor shaft. A ball nut arrangement is interposed between the hollow motor shaft and the drive shaft for converting the rotational movement of the hollow motor shaft into the axial movement of the drive shaft.
In such an electric power steering device, the electric motor must have a required level of performance for the electric power steering device to operate in a satisfactory manner. However, once the power steering device is fully assembled, it is not practical to replace or repair the electric motor even if the electric motor is found to be faulty or otherwise incapable of producing a required level of performance. Therefore, it has been a common procedure in the manufacturing of electric power steering devices to test the operation of the electric motor as a part of the assembling process.
FIG. 6 illustrates a conventional electric power steering device which comprises a yoke 1 consisting of an axially elongated tube member serving also as a part of the housing for the power steering device, and a gear casing which accommodates a rack and pinion gear device and coaxially attached to the yoke 1 via a radial flange to form the rest of the housing for the power steering device. The power steering device normally extends laterally across the vehicle body which is not shown in the drawing, and is attached to the vehicle body via a bracket 2a formed in the gear casing 2 and a mounting bracket 13 partly surrounding a small diameter portion of the yoke 1. These brackets 2a and 13 are normally secured to the vehicle body by fasteners such as threaded bolts.
The yoke 1 and the gear casing 2 thus jointly form the housing for the power steering device, and coaxially receive therein a drive shaft 3 which is axially slidable so as to steer the right and left front wheels via the tie rods attached to either axial end thereof. The drive shaft 3 is connected to the rack of a rack and pinion gear device, and the pinion thereof is attached to a pinion shaft 4 which is connected to a steering shaft.
An intermediate part of the drive shaft 3 is coaxially received in a hollow motor shaft 5. A motor armature core 6a and a commutator 6b are attached to the outer circumferential surface of the hollow motor shaft 5 so that the armature core 6a and the hollow motor shaft 5 rotate integrally together. A ball nut mechanism is interposed between the inner circumferential surface of the hollow motor shaft 5 and the outer circumferential surface of the drive shaft 3 to convert the rotational movement of the hollow motor shaft 5 to the axial movement of the drive shaft 3. The rotational torque produced from the motor armature 6 is converted into the axial assist force for the drive shaft 3 so that the effort required for turning the steering wheel attached to the pinion shaft 4 may be reduced.
According to this electric power steering device, the outer diameter of the ball nut is greater than the outer diameter of the drive shaft 3. Further, because of the need to restrict the axial movement of the drive shaft 3 within a prescribed range at an open end of the yoke 1, and the need to seal the annular gap between the open end of the yoke 1 and the outer circumferential surface of the drive shaft 3, the inner diameter of the open end of the yoke 1 was conventionally smaller than the outer diameter of the ball nut. Therefore, conventionally, there was some difficulty in directly measuring the output torque of the electric motor. To overcome this difficulty, conventionally, the thrust force of the drive shaft 3 was measured instead of directly measuring the output torque of the electric motor.
However, the thrust force of the drive shaft 3 does not necessarily represent the output torque of the electric motor because of the friction loss in the ball nut mechanism, manufacturing errors in the ball nut mechanism, and the machining errors in the drive shaft. Therefore, conventionally, to be able to accurately determine the torque output of an electric motor for an electric power steering device of this type, it was necessary either to measure the mechanical loss that may be present in the ball nut mechanism or to test the motor armature in a special jig incorporated with permanent magnets and a yoke. In either case, extra preparatory steps and extra testing steps must be added to the manufacturing process, and this leads to the reduction in the manufacturing efficiency. Also, the test precision may still not be very high because of a number of factors which arise from the fact that the motor output torque has to be measured indirectly.